It is known to use fluid sensors to measure the composition and/or flow characteristics of a fluid. Such fluid sensors are often referred to as multiphase meters. Known multiphase meters comprise a base pipe defining a fluid flow path internally thereof surrounded by a concentrically arranged open-ended generally cylindrical metallic cavity member. The base pipe is substantially transparent to radio frequency (RF) electromagnetic radiation. The cavity member defines a resonant cavity for a RF electromagnetic field which extends through the base pipe and across the fluid flow path. In known multiphase meters the base pipe may be formed of polyvinyl chloride (PVC) or polyether ether ketone (PEEK) and the cavity member is formed of brass. Such known multiphase meters are configured to detect a resonant peak in the frequency spectrum of the RF electromagnetic field and to extract the composition and/or flow characteristics of fluid in the fluid flow path from characteristics of the resonant peak.
It is well known that the strength of an RF electromagnetic field varies across a resonant cavity. Consequently, when a non-homogeneous fluid is present in the fluid flow path, different fluid components (e.g. water, oil or gas) present in the fluid may be located or flow through regions having significantly different RF electromagnetic field strengths. If the different fluid components move position across the fluid flow path this can make measurements of the composition and/or flow characteristics of the fluid in the fluid flow path more difficult and/or less accurate. Accordingly, in known multiphase meters, the cavity member is generally separated from the base pipe so as to define a resonant cavity which is significantly greater in cross-section than the fluid flow path for improved uniformity of the RF electromagnetic field strength across the fluid flow path. Consequently, known multiphase meters have an annular outer cavity region defined between an outer surface of the base pipe and an inner surface of the cavity member.
In known multiphase meters the annular outer cavity region is filled with air or water. Examples of such known multiphase meters are described in S. Al-Hajeri, S. R. Wylie, R. A. Stuart and A. I. Al-Shamma'a, “An electromagnetic cavity sensor for multiphase measurement in: the oil and gas industry”, Journal of Physics: Conference Series 76 (2007) 012007; in S. Al-Hajeri, S. R. Wylie, A. Shaw and A. I. Al-Shamma'a “Real time EM waves monitoring system for oil industry three phase flow measurement”, Journal of Physics: Conference Series 178 (2009) 012030; in S. R. Wylie, A. I. Al-Shamma'a, A. Shaw and S. Al-Hajeri, “Electromagnetic cavity sensors for multiphase measurement”, Exploration and Production Oil and Gas Review, Volume 9, Issue 1; and in Finnish patent document no. FI834892.
The use of a fluid sensor comprising an air-filled outer cavity region may be problematic especially in a high pressure environment because it may be necessary for the cavity member to be configured to withstand high external pressures. Similarly, if the cavity member is surrounded by a casing for protection in a high pressure environment, it may be necessary for the casing to be configured to withstand high external pressures. In a subsea environment, it may also be important to provide the air-filled outer cavity region with high pressure seals to prevent water ingress. Moreover, for high internal fluid pressures within the fluid flow path it may be necessary for the base pipe to be configured to withstand the high internal fluid pressures for similar reasons.
The use of a fluid sensor comprising a water-filled outer cavity region may also be problematic because water has a relatively high conductivity. This may result in absorption of the RF electromagnetic field in the water-filled outer cavity region and may make it difficult to detect a resonant peak in the RF electromagnetic field. This may make measurements of the composition and/or flow characteristics of the fluid in the fluid flow path more difficult and/or less accurate. Moreover, although water is generally much less compressible than air, if the external and/or internal fluid pressures are sufficiently high, it may still be necessary for the cavity member and/or the base pipe to be configured to withstand high external and/or high internal fluid pressures. To prevent any reduction in structural integrity of the fluid sensor, it may also be important to provide the water-filled outer cavity region with high pressure seals to prevent water egress.
In addition, for the case of known fluid sensors having air- or water-filled outer cavity regions where the base pipe comprises a polymeric material, pressurised fluids, particularly gasses, can migrate from the fluid flow path through the polymeric material over time and accumulate in the outer cavity region. This can be problematic if the fluid sensor is later subjected to a reduction in internal or external fluid pressure because such depressurisation may lead to expansion of the accumulated fluids and may result in deformation or, in the worst case, structural failure of the fluid sensor.